The petroleum industry has long been interested in the production of "synthetic" liquid fuels from non-petroleum solid fossil fuel sources. It is hoped that economic non-petroleum sources of liquid fuel will help the petroleum industry to meet growing energy requirements and decrease dependence on foreign supplies.
Coal is the most readily available and most abundant solid fossil fuel, others being tar sands and oil shale. The United States is particularly richly endowed with well distributed coal resources. Additionally, in the conversion of coal to synthetic fuels, it is possible to obtain liquid yields of about three to four barrels per ton of dry coal, or about four times the liquid yield/ton of other solid fossil fuels such as tar sands or shale, because these resources contain a much higher proportion of mineral matter.
Despite the continued interest and efforts of the petroleum industry in coal liquefaction technology, further improvements are necessary before it can reach full economic status. Maximizing the yield of coal liquids is important to the economics of coal liquefaction.
The present invention relates to an improved process for converting coal to liquid hydrocarbon products in a catalytic hydroconversion process. The improvement relates to a coal pretreatment stage comprising subjecting the coal to aqueous carbon monoxide in the presence of a promotor under specific pressure and temperature conditions. Such pretreatment improves the reactivity of the coal in the subsequent liquefaction stage of the overall process. Various promoters enhance the hydrogenation and depolymerization of the coal during pretreatment. The promoters disclosed herein include certain organic acids and salts of organic acids. 2. Description of the Prior Art
The known processes for producing liquid fuels from coal can be grouped into four broad categories: direct hydrogenation, donor solvent hydrogenation, Fischer-Tropsch synthesis (via gasification), and pyrolysis (see Kirk Othmer - Fuels). The present invention falls into the category of direct hydrogenation.
The direct hydrogenation of coal in the presence of solvent and catalyst was first developed in Germany prior to World War II. In such a process, a slurry of coal in a suitable solvent was reacted in the presence of molecular hydrogen at an elevated temperature and pressure.
A number of previous co-assigned patents disclose coal liquefaction processes utilizing hydroconversion catalysts which are micron sized particles comprised of a metal sulfide in a carbonaceous matrix. These catalysts are generally formed from certain soluble or highly dispersed organometallic compounds or precursors. These precursors are converted into catalyst particles by heating in the presence of an hydrogen containing gas. The catalyst particles are highly dispersed in the feed being treated during hydroconversion. Among the various patents in this area are U.S. Pat. No. 4,077,867; U.S. Pat. No. 4,094,765; U.S. Pat. No. 4,149,959; U.S. Pat. No. 4,298,454; and U.S. Pat. No. 4,793,916. Other patents disclose catalysts similar to the above except that the catalytically active metal compound is supported on finely divided particles of solid metals and metal alloys, for example as disclosed in U.S. Pat. Nos. 4,295,995 and 4,357,229.
The conversion of coal in aqueous/CO is well known, dating back to Fischer and Schrader in 1921 (F. Fisher & H. Schrader, Bennst. Chem., 2, 257, 1921). Several liquefaction processes, including the U.S. Bureau of Mines COSTEAM process (H. R. Appell, E. C. Moroni, R. D. Miller, Energy Sources, 3, 163, (1971), have been developed based on using aqueous/CO or aqueous/syngas at 750.degree.-850.degree. F. in the primary conversion block for lignites (low rank coals).
An object of the present invention is to provide an improved process for the conversion of carbonaceous solids such as coal in order to produce valuable liquid hydrocarbonaceous products.
A further object of the present invention is to provide an improved process for producing liquid hydrocarbonaceous products from coal, the improvement comprising utilizing a pretreatment step wherein the coal, slurried in an aqueous or organic solvent, is subjected to reaction with carbon monoxide in the presence of a promoter compound to enhance hydrogenation and/or depolymerization of the coal.
A particular object of the present invention is to pretreat coal in a specific temperature range to generate a more reactive coal for coal liquefaction, thereby obtaining more products, with higher selectivity to liquids over gases.
Another object of the present invention is to improve the efficiency in the utilization of molecular hydrogen in the transformation of coal to valuable liquids.
Another object of the present invention is to reduce the investment necessary for building a coal liquefaction plant and/or to increase the capacity of such a plant by permitting a lower average residence time for coal treatment and conversion.
Additional advantages of the present coal conversion process will become apparent in the following description.